A well capable of producing oil or gas that is deep enough will typically have a well structure to provide support for the borehole and isolation capabilities for different formations. Typically, the well structure includes an outer structure, such as a conductor housing at the surface, that is secured to conductor pipe that extends a short depth into the well. A wellhead housing is landed in the conductor housing with an outer or first string of casing extending from the wellhead and through the conductor to a deeper depth into the well. Depending on the particular conditions of the geological strata above the target zone (typically, either an oil or gas producing zone or a fluid injection zone), one or more additional casing strings (e.g., production casing, casing, tubing, production tubing, etc.) will extend through the outer string of casing to increasing depths until the well is cased to its final depth. Each string of casing is supported at the upper end by a casing hanger that lands in and is supported by the wellhead housing, each set above the previous one. Between each casing hanger and the wellhead housing, a casing hanger seal assembly is set to isolate each annular space between strings of casing. The last, and innermost, string of casing extends into the well to the final depth and is referred to as the production casing. The strings of casing between the outer casing and the production casing are typically referred to as intermediate casing strings.
When drilling and running strings of casing in the well, it is critical that the operator maintain pressure control of the well. This is accomplished by establishing a column of fluid with predetermined fluid density inside the well that is circulated down into the well through the inside of the drill string and back up the annulus around the drill string to the surface, for example. This column of density-controlled fluid balances the downhole pressure in the well. A blowout preventer system (BOP) is also used to as a safety system to ensure that the operator maintains pressure control of the well. The BOP is located above the wellhead housing and is capable of shutting in the pressure of the well, such as in an emergency pressure control situation.
After drilling and installation of the casing strings, the well is completed for production by installing a string of production tubing that extends to the producing zone within the production casing, for example. Perforations are made in the production casing to allow fluids to flow from the formation into the productions casing at the producing zone. At some point above the producing zone, a packer seals the space between the production casing and the production tubing to ensure that the well fluids flow through the production tubing to the surface. The tubing is supported by a tubing hanger assembly that lands and locks above the production casing hanger.
Various arrangements of production control valves are arranged at the wellhead in an assembly generally known as a tree, which is generally either a vertical tree or a horizontal tree. With a vertical tree, after the production hanger and production tubing are installed in the wellhead housing, the BOP is removed and the vertical tree is locked and sealed onto the wellhead. The vertical tree has one or more production bores containing actuated valves that extend vertically to the respective lateral production fluid outlets in the vertical tree. The production bores and production valves are thus in-line with the production tubing.
With a vertical tree, the tree may be removed while leaving the completion (the production tubing and hanger) in place. However, if it is necessary to pull the completion, the vertical tree must be removed and replaced by a BOP, which involves setting and testing plugs or relying on downhole valves, which may be unreliable by not having been used or tested for a long time. Moreover, removal and installation of the tree and BOP assembly generally requires robust lifting equipment, such as a rig, that have high daily rental rates, for instance. The well is also in a vulnerable condition while the vertical tree and BOP are being exchanged and neither of these pressure-control devices is in position, which is a lengthy operation that usually involves plugging and/or killing the well.
Instead of vertical trees, trees with the arrangement of production control valves offset from the production tubing, generally called horizontal trees, can be used. One type of horizontal tree is a Spool Tree™ which is shown and described in U.S. Pat. No. 5,544,707, hereby incorporated herein by reference for all purposes. A horizontal tree also locks and seals onto the wellhead housing; but the tubing hanger, instead of being located in the wellhead, locks and seals in the tree bore. After the tree is installed, the tubing string and tubing hanger are run into the tree using a tubing hanger running tool. The production port extends through the tubing hanger and seals prevent fluid leakage and production fluid flows into the corresponding production port in the tree. A locking mechanism above the production seals locks the tubing hanger in place in the tree. With the production valves offset from the production tubing, the production tubing hanger and production tubing may be removed from the tree without having to remove the horizontal tree from the wellhead housing. A problem with horizontal trees, however, is that if the tree needs to be removed, the entire completion must also be removed, which takes considerable time and also involves setting and testing plugs or relying on downhole valves, which may be unreliable by not having been used or tested for a long time. Additionally, because the locking mechanism on the tubing hanger is above and blocks access to the production port seals, the entire completion must be pulled, should the seals requiring servicing.
To manage expected maintenance costs, which are especially high for an offshore well, an operator typically selects equipment best suited for the type of maintenance he or she expects will be required. For example, a well operator must predict whether there will be a greater need in the future to pull the tree from the well for repair, or pull the completion, either for repair or for additional work in the well. Depending on the predicted maintenance events, an operator must decide whether the horizontal or vertical tree, each with its own advantages and disadvantages, is best suited for his or her purpose. For instance, with a vertical tree, it is more efficient to pull the tree and leave the completion in place. However, if the completion needs to be pulled, the tree must be pulled as well, increasing the time and expense of pulling the completion. Just the opposite is true for a horizontal tree, where it is more efficient to pull the completion, leaving the tree in place. However, if the tree needs to be pulled, the entire completion must be pulled as well, increasing the time and expense of pulling the tree. The life of the well could easily span 20 years and it is difficult to predict at the outset which capabilities are more desirable for maintenance over the life of the well. Thus, an incorrect prediction could greatly increase the cost of production over the life of the well.